1. Field of the Invention
This invention relates to a thermochromic material comprising, as indispensable components, an electron-donating, chromatic organic compound, a compound having a phenolic hydroxyl group and a compound selected from the group consisting of higher aliphatic monatomic alcohols and higher aliphatic monatomic acid alcohol esters.
2. Description of the Prior Art
Metal complex crystals having specific thermochromic properties have heretofore been used as thermochromic materials. However, in these metal complex crystals the metachromatic temperature range is substantially from 50.degree. C. to about 300.degree. C. or higher, and in most of these conventional thermochromic materials the methacromatism is caused to occur at temperatures exceeding 100.degree. C., and none of them exhibit methacromatism at ordinary ambient temperatures. Accordingly, their applications are limited. Further, in these complexes, the color or methachromatism-causing temperature cannot be freely chosen, but is inherently determined by the properties of the complexes per se.
More specifically, the number of substances undergoing metachromatism at temperatures not exceeding 100.degree. C. is limited to 2 or 3. For instance, in the case of Ag.sub.2 HgI.sub.4, the metachromatism from yellow to orange occurs at 50.degree. C., and in the case of Cu.sub.2 HgI.sub.4 metachromatism from red to brown is brought about at 70.degree. C. Of course, the kind of color cannot be optionally chosen and there is no marked difference between colors before and after metachromatism.
Moreover, since these metal complex crystals are not light-transmitting, it is impossible to optionally hide or reveal the background using these metal complex crystals.
Still further, since most of these complex salts exhibiting metachromatic characteristics contain heavy metals (especially those complex salts exhibiting metachromatism at a temperature of less than 100.degree. C.) such as mercury as mentioned above, care must be taken so as to prevent accidents, especially environmental pollution, when such complex salts are employed. From this viewpoint, it is impossible to employ a large quantity of such complex salts without difficulty.
As another example of a conventional thermochromic material, there can be mentioned liquid crystals in which metachromatism occurs at temperatures ranging from -10.degree. C. to +200.degree. C. However, the number of liquid crystals undergoing metachromatism at a temperature not exceeding 0.degree. C. is very limited, namely 1 or 2. As in the case of the foregoing metal complex crystals, the color or metachromatism-causing temperature cannot freely be chosen but is determined by the properties of the liquid crystals per se. In other words, a substance having a desired color and a desired metachromic temperature must be chosen from substances heretofore synthesized or it must be newly synthesized.
Since these metachromatic compounds are chemically very sensitive, their properties are readily degraded upon contact with other substances. Further, a black underground is required in the case of cholesteric liquid crystals and therefore, only materials of a dark hue are obtained. Moreover, these compounds are very expensive. In view of the foregoing defects, use of these compounds as thermochromic materials includes various commercial and industrial limitations and difficulties, and their fields of application are very limited.
Many uses in which the phenomenon of a color change caused by a change in temperature is utilized have been considered, but materials suitable for such a purpose have not been found and development of thermochromic materials excellent in thermochromic characteristics have been greatly demanded in the art.